Background Swingers, heterosexuals who, as couples, practice mate swapping or group sex with other couples or heterosexual singles, are at risk for sexually transmitted infections (STIs). Therefore, the aim of this study was to assess changes in sexual behaviour and STI testing behaviour, as well as predictors of STI testing. Methods Two cross-sectional studies were performed, using the same internet survey in 2011 and 2018. For trend analysis, sexual behaviour and STI testing behaviour were used. Socio-demographics, swinger characteristics, sexual behaviour, and psycho-social variables were used to assess predictors of STI testing in the past year, using multivariable regression analysis. Results A total of 1173 participants completed the survey in 2011, and 1005 in 2018. Condom use decreased for vaginal (73% vs. 60%), oral (5% vs. 2%), and anal sex (85% vs. 75%). STI positivity was reported in 23% and 30% of the participants, respectively, although testing for STI was comparable between both years (~65%). The following predictors of STI testing were significant: being female (OR = 1.9, 95%CI: 1.2–2.9), having a high swinging frequency (>12 times a year, OR = 3.7, 95%CI: 1.9–7.3), swinging at home (OR = 1.6, 95%CI: 1.0–2.7), receiving a partner notification (OR = 1.7, 95%CI: 1.2–2.6), considering STI testing important (OR = 4.3, 95%CI: 2.2–8.5), experiencing no pressure from a partner to test (OR = 0.6, 95%CI: 0.3–0.9), partners test for STI regularly (OR = 10.0, 95%CI: 6.2–15.9), perceiving STI testing as an obligation (OR = 2.1, 95%CI: 1.3–3.5), experiencing no barriers such as being afraid of testing (OR = 1.9, 95%CI: 1.2–3.1), limited opening hours (OR = 1.6, 95%CI: 1.0–2.4), and forgetting to plan appointments (OR = 3.0, 95%CI: 2.0–4.6). Conclusions Swingers exhibit self-selection for STI testing based on their sexual behaviour. However, STI prevention efforts are still important considering the increasing numbers of reported STIs, the decreased use of condom use, and the one-third of swingers who were ...